(1) Field of the Invention
The present invention relates to a device and method for measuring a Specific Absorption Rate (SAR) in human bodies exposed to electromagnetic waves from antennas of portable telephones, radio frequency equipment, etc. positioned in the vicinity of human bodies.
(2) Description of the Related Art
One of the basic standard guidelines for protecting human bodies from electromagnetic waves is known as the Specific Absorption Rate (SAR). This represents a value of energy absorption per unit mass when a human body is exposed to electromagnetic waves, and the tolerable value is indicated in, for example, "Guidelines for protection of Humans in Utilization of Electromagnetic Waves" proposed by "Electro-Communication Technology Study Council" of Japanese Ministry of posts and Telecommunications.
Conventionally, there has been a study wherein, by using a phantom (biological body model) having the permittivity and the permeability equivalent to those of biological bodies such as human bodies, the SAR has been measured from the incident magnetic fields on the phantom. This has been proposed by N. Kuster and Q. Balzano under the title "Energy Absorption Mechanism by Biological Bodies in the Near Field of Dipole Antennas Above 300 MHz" in IEEE Transaction on Vehicular Technology, Vol. 41, No. 1, 1992, pp 17-23.
In the conventional SAR measuring system which is adopted in the above proposal and which is illustrated in FIGS. 1A and 1B attached hereto, the electromagnetic waves from half-wave dipole antenna 4 are incident perpendicularly on the phantom 5. The distance between the antenna 4 and the phantom 5 is represented by d. Where the real part of the permittivity of the phantom is represented by .epsilon., the conductivity by .sigma., the mass density by .rho., and the incident magnetic field on the front surface of phantom by Hs, the SAR may be expressed by the following equation: ##EQU1## wherein .omega. represents an angular frequency, .GAMMA. represents a reflection coefficient, .alpha. represents a correction coefficient, and .mu..sub.o =4.pi..times.10.sup.-7.
The incident magnetic fields are proportional to the antenna currents so that the SAR can be expressed also by the antenna currents. In the half-wave dipole antenna, the SAR maximum value of the phantom is at a portion corresponding to the location of the driving point so that, where the antenna current at the driving point is assumed to be I, the incident magnetic fields may be expressed by the following equation: EQU Hs=I/(2.pi.d) (2)
The current I is calculated from the magnetic fields 8 radiating from the antenna 4 by using a magnetic field receiving loop probe 7. The probe 7 is placed in such a way that the loop surface becomes perpendicular with respect to the X-axis at a location away by a distance t from the driving point of the probe. At this location, the radiating magnetic fields 8 have only X-direction components.
Normally, the characteristic impedance of the human body is of a lower impedance than 120.pi. which is the characteristic impedance in air. Thus, the electromagnetic waves radiating from the antenna reflect at the front surface of the phantom. FIG. 3 shows incident waves and reflected waves when the electromagnetic fields are incident on the phantom 5 at an incident angle of .theta. and the magnetic fields radiating from the antenna 4. The magnetic fields and the electric fields of the incident waves are represented by Hin and Ein, respectively, the reflected waves are represented by Hr and Er, and the radiating magnetic fields from the antenna are represented by Hi. The X-axis direction component Hr(X) and the Y-axis direction component Hr(Y) of the reflected magnetic fields at the location where the probe is placed may be expressed by the following equations, respectively: EQU Hr(X)=Hr.sup..multidot. sin .theta. (3) EQU Hr(Y)=Hr.sup..multidot. cos.theta. (4)
Therefore, the magnetic fields Hp of the X-axis direction which the probe receives may be expressed by the following equation: EQU Hp=Hi-Hr(X) (5)
Where the phantom is located remotely, the reflected magnetic fields attenuate with distance, resulting in a small effect on the phantom. However, where the phantom is located in the vicinity of the antenna, the effect from the reflected waves can no longer be ignored.
When the distance d between the antenna and the phantom is varied, the relation between the reflected magnetic fields Hr(X) in the X-direction and the radiating magnetic fields Hi will be as shown in FIG. 4. When the distance d becomes smaller than 3 cm, the reflected magnetic fields increase to more than 25%. Thus, the problem in the probe arrangement as above is that the probe receiving the magnetic fields for the calculation of the antenna currents results in being measured smaller than the actual value, which leads to a large SAR error.